Pasteurella multocida is a gram-negative, non-motile, rod shaped, facultative anaerobe which is isolated from a wide range of animals and birds from all over the world.
The P. multocida isolates are classified into five serogroups (A, B, D, E and F) based on capsular antigens and 16 serotypes by somatic antigens (Rimier and Rhoades, 1989). Serogroup A is most commonly associated with fowl cholera in birds followed by serogroup D (Rhoades and Rimler, 1989). Among the isolates, serogroup F strains are predominantly isolated from poultry and turkeys, but rarely from calves (Shewen and Conlon, 1993; Catry et al., 2005). In pigs, atrophic rhinitis and pneumonia are primarily associated with serogroups D and A which express dermonecrotizing toxin (Dungworth, 1985). On the other hand P. multocida serogroups B and E are usually associated with hemorrhagic septicemia in cattle and water buffaloes in tropical and sub-tropical regions of Africa and Asia (Carter and de Alwis, 1989; Rimler and Rhoades, 1989; Shewen and Conlon, 1993). In contrast P. multocida serogroups B and E are rarely isolated North America cattle population (Confer, 1993). More than 92% of P. multocida isolated from the US cattle which cause severe suppurative bronchopneumonia belong to serotype A:3 (Ewers et al., 2006; Confer et al., 1996; Weekley et al., 1998). P. multocida infection in calves results in significant production yield losses and mortality (Ewers et al., 2006; Confer et al., 1996; Dalgleish, 1989; Weekley et al., 1998). Furthermore, P. multocida is often associated with bovine respiratory disease complex (BRDC) along with Mannheima haemolytica and Histophilus somni. From 2001, bovine pneumonic pasteurellosis due to P. multocida infection has increased in the UK cattle population. In many UK cases, P. multocida infections exceeded the number of outbreaks caused by M. haemolytica induced bovine bacterial pneumonia (Veterinary Laboratories Agency, 2007). Worldwide, P. multocida serogroup A isolates are one of the major pathogens associated with BRDC (Frank, 1989; Rimler and Rhoades, 1989).
P. multocida isolates associated with BRDC have numerous virulence or potential virulence and virulence-associated factors like adhesins and filamentous hemagglutinin which aid in adherence and colonization, iron acquisition proteins and transport systems, extracellular enzymes such as neuraminidase, endotoxin (lipopolysaccharide, LPS), polysaccharide capsule and a variety of outer membrane proteins (OMPs). Immunity of cattle against respiratory pasteurellosis is poorly understood; however some reports indicate that high serum antibodies against P. multocida OMPs are important for enhancing resistance against this bacterium.
There are a few commercial vaccines currently available against P. multocida for use in cattle. These vaccines are predominately traditional bacterins and a live streptomycin-dependent mutant. However, the field efficacy of these vaccines is questionable and none of the vaccines afford reliable protection. Therefore, there remains a need for safe and effective vaccines to protect cattle against P. multocida infections.